Hitherto, a surface acoustic wave device capable of functioning as a filter element or an oscillator used for a mobile phone or the like and an elastic wave device, such as a Lamb wave element or a thin film resonator (FBAR: Film Bulk Acoustic Resonator), using a piezoelectric thin film have been known. As the elastic wave devices described above, a device has been known which is formed in such a way that after a piezoelectric substrate which transmits an elastic wave and a support substrate having a smaller linear thermal expansion coefficient (hereinafter referred to as “thermal expansion coefficient”) than that of this piezoelectric substrate are bonded to each other to form a composite substrate having a size of several inches, many comb-teeth electrodes are formed on this composite substrate using a photolithographic technique, and cutting is then performed by dicing. Since the change in size of the piezoelectric substrate caused by the change in temperature can be suppressed by using the composite substrate as described above, frequency characteristics as the elastic wave device can be stabilized. For example, in Patent Document 1, there has been proposed an elastic wave device having the structure in which an LT substrate (LT is an abbreviation of lithium tantalate) which is a piezoelectric substrate and a silicon substrate which is a support substrate are adhered to each other with an adhesive layer which is formed from an epoxy adhesive. After the elastic wave device as described above is mounted on a ceramic substrate by flip-chip bonding with gold balls interposed therebetween and is then encapsulated with a resin, this ceramic substrate is mounted on a printed circuit board using electrodes provided on a rear surface of the ceramic substrate with lead-free solder interposed therebetween. In addition, instead of using gold balls, the elastic wave device as described above may be mounted on a ceramic substrate with balls formed of lead-free solder in some cases. Also in this case, the lead-free solder is melted and re-solidified in a reflow step when mounting is performed.
Patent Document 1: JP 2007-150931 A
However, in a conventional elastic wave device, cracks are generated in some cases after the reflow step is finished, and there has been a problem in that the production yield is low. The reason the problem described above occurs is that, in a piezoelectric substrate and/or a support substrate having an anisotropic thermal expansion coefficient and an anisotropic elastic modulus (Young's modulus), a portion is present in which the difference in thermal expansion coefficient is large and the elastic modulus is low. That is, it is believed that in the portion as described above, since a large thermal stress is generated at a high temperature due the difference in thermal expansion coefficient, a thermal strain induced by this thermal stress is increased because of the low elastic modulus, and as a result, cracks are generated. In addition, also in a step other than the reflow step which is performed when an elastic wave device is mounted, a high-temperature process at a temperature of 250° C. to 300° C. may be performed in some cases on a composite substrate which still has a size of several inches, and in this case, cracks may also be generated by the same reason as described above.